Exhaust systems for marine engines generally include an exhaust manifold connected to the engine at each row (or “bank”) of engine cylinders, and a corresponding exhaust conduit coupled to the exhaust manifold for directing exhaust gases from the manifold to an exhaust outlet. In conventional exhaust systems, the exhaust conduit includes a horizontally oriented catalytic converter assembly having a catalyst that removes harmful emissions from the exhaust gases before being expelled through the exhaust outlet.
Exhaust systems can experience extremely high temperatures during use. For example, the core temperature of a catalytic converter in a conventional exhaust system can reach upwards of 1,000 degrees Fahrenheit (° F.) or more. For safety purposes, the U.S. Coast Guard requires that exterior surface temperatures of marine engine exhaust systems be maintained below 200° F. Accordingly, components of conventional marine engine exhaust systems, including the catalytic converter assemblies, are often liquid-cooled to ensure safe and compliant operating temperatures.
Referring to FIG. 1, an inlet end portion of an exhaust conduit 1 of a conventional marine engine exhaust system is shown. The exhaust conduit 1 includes an inner (or “exhaust”) tube 2 and an outer tube 3 that surrounds and is spaced radially outward from the inner tube 2 so as to define a passage 4 through which cooling liquid L, such as water, is directed from an inlet hose 12. The inner and outer tubes 2, 3 are pinched together at their inlet ends to form a cylindrical pinched portion 5. A first weld 7 joins the inner and outer tubes 2, 3 together at the pinched portion 5, and also joins to a conduit flange 6. A second weld 8 joins an outer surface of the outer tube 3 to the conduit flange 6. The exhaust conduit 1 is then coupled to an exhaust manifold 9 by coupling the conduit flange 6 to a corresponding manifold flange 10, so that the exhaust conduit 1 may direct hot exhaust gases G toward an exhaust outlet (not shown).
The pinched portion 5 of the exhaust conduit 1 is formed by deforming the inlet end portion of the inner tube 2 radially outward so as to define a conical flared portion 11 having a non-uniform cross-sectional diameter that varies along a length of the inner tube 2 at its inlet end portion. This deformation step results in a decreased (or thinned) wall thickness of the inner tube 2 at its inlet end portion, for example at the flared portion 11, thereby undesirably weakening this portion of the inner tube 2. Further, this configuration of the exhaust conduit 1 provides an inlet end of the cooling passage 4 with a tapered configuration that tends to trap precipitated salts and other particulate from cooling liquid L, particularly when the cooling liquid L is in the form of water. It has been observed that buildup of these salts and particulate, in combination with the decreased wall thickness of the inner tube 2, may disadvantageously result in corrosion and eventual cracking of at least the inner tube 2 at its inlet end portion. Such cracking can cause cooling liquid L to leak from the cooling passage 4, thereby depriving the exhaust conduit 1 of necessary cooling.
An additional disadvantage of the known configuration shown in FIG. 1 is that the conduit flange 6, which directly contacts the extremely hot exhaust manifold flange 10, is deprived of direct cooling. As shown, the flange 6 is separated from the cooling passage 4 by the circumferential pinched portion 5. As such, the cooling liquid L is unable to directly contact and cool the flange 6, and the flange 6 thus may exhibit extremely high and unsafe surface temperatures compared to surface temperatures of neighboring components of the exhaust system.
Accordingly, there is a need for improvements to known marine engine exhaust systems to address these and other shortcomings.